The present invention relates to an electrophotographic imaging apparatus, and more particularly to systems and methods for characterizing laser beam process direction position errors.
In electrophotography, a latent image is created on the surface of an electrostatically charged photoconductive drum by exposing select portions of the drum surface to laser light. Essentially, the density of the electrostatic charge on the surface of the drum is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the surface of the drum to toner, which contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the drum surface in a manner that corresponds to the electrostatic density altered by the laser beam. Subsequently, a print medium, such as paper, is given an electrostatic charge opposite that of the toner and is pressed against the drum surface. As the medium passes the drum, the toner is pulled onto the surface thereof in a pattern corresponding to the latent image written to the drum surface. The medium then passes through a fuser that applies heat and pressure thereto. The heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the fuser pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
In order to produce an accurate representation of an image to be printed, it is necessary for the laser to write to the drum in a scan direction, which is defined by a straight line that is perpendicular to the direction of movement of the print media relative to the drum (the process direction). Moreover, the laser should be capable of writing a line of evenly spaced, print elements (Pels) on the surface of the drum. However, manufacturing tolerances, imperfections of optical devices in the optical system, and the inherent configuration of the printhead cause imperfections in the spacing between written Pels along a scan line, which is referred to herein as scan line nonlinearity. Particularly, the velocity of the laser beam varies across the scan line, which typically causes consecutive Pels to be written farther apart near the ends of the scan line, and closer together near the middle portion of the scan line.
The prior art has attempted to compensate for scan line nonlinearity by providing carefully aligned and calibrated optics. For example, it is known to use an f-theta lens in the optical system of a printhead to optically modify the laser beam scan path to attempt to achieve a more constant linear velocity. However, the increased precision required by the optical elements adds significantly to the cost of the printhead. Even with precisely manufactured and aligned optics, the degree to which laser beam scan linearity may be corrected is limited by several factors, including component tolerances. Moreover, nonlinearity of laser beam scan velocity can occur even in precisely calibrated optical systems due to component aging and/or operational influences such as temperature changes.